1. Field of the Invention
This specification relates to a method of fabricating a thin film solar cell, and particularly, to a method of fabricating a thin film solar cell capable of reducing the fabrication costs and enhancing spatial efficiency.
2. Description of the Related Art
Generally, a solar cell is a device for converting solar energy into electrical energy. This solar energy has a junction between a p-type semiconductor and an n-type semiconductor, and has the same basic structure as a diode.
An operation of this solar cell will be explained as follows.
Generally, the solar cell consists of a PN junction diode of a large area. For photovoltaic energy conversion, the solar cell is required to have a condition that electrons are asymmetrically disposed in a semiconductor structure. More concretely, a p-type semiconductor region has a low electron density and a high hole density, and an n-type semiconductor region has a high electron density and a low hole density. Accordingly, a PN junction diode in a thermal equilibrium state has unbalanced charges due to diffusion by a concentration gradient. This may cause formation of an electric field, thereby resulting in no diffusion of carriers. When light having energy more than a band gap (an energy difference between a conduction band and a valence band) is applied to the diode, electrons having received the light energy become excited from the valence band to the conduction band. The electrons which have become excited to the conduction band freely move, and holes are generated at positions of the valence band, the positions from which the electrons have passed out. This is called ‘excess carriers’, and these ‘excess carriers’ are diffused in the conduction band or the valence band due to a concentration difference. Here, electrons which have become excited in a p-type semiconductor and holes formed in an n-type semiconductor are called ‘minority carriers’, respectively. On the other hand, the existing carriers inside the p-type or n-type semiconductor before a junction (holes of the p-type semiconductor and electrons of the n-type semiconductor) are called ‘majority carriers’.
The ‘majority carriers’ are hindered from moving due to an energy barrier which occurs by an electric field. However, the electrons, the ‘minority carriers’ of the p-type semiconductor can move to the n-type semiconductor, respectively.
A potential drop is generated inside the PN junction diode due to diffusion of the minority carriers. And, the PN junction diode may serve as a solar cell when an electromotive force generated from two ends of the PN junction diode is connected to an external circuit.
The solar cell may be largely categorized into a silicon-based solar cell, a compound-based solar cell and an organic-based solar cell according to a used material.
The silicon-based solar cell is categorized into a monocrystalline silicon-based solar cell, a polycrystalline silicon-based solar cell and an amorphous silicon-based solar cell according to a phase of a semiconductor.
The solar cell is also categorized into a bulk-type solar cell and a thin film solar cell according to a thickness of a semiconductor layer. A thickness of a semiconductor layer of the thin film solar cell corresponds to several μm-several tens of μm.
Among these various types of solar cells, a bulk-type solar cell having high energy conversion efficiency and low fabrication costs is being widely utilized for terrestrial electricity.
However, according to recent high demands for the bulk-type solar cell, raw materials of the bulk-type solar cell become deficient. This may cause the bulk-type solar cell to have a high price.
For low fabrication costs and massive productions of the solar cell for terrestrial electricity, required is a thin film solar cell capable of reducing the amount of a raw material, silicon into 1/100 of the currently-used amount.
The thin film solar cell has an advantage that a large area is implemented more easily than the bulk-type solar cell. However, the thin film solar cell has a disadvantage that energy conversion efficiency is degraded as an area is increased due to a resistance of a transparent electrode of a light receiving surface.
In order to solve this problem, an integrated-type thin film solar cell has been developed. The integrated-type thin film solar cell has a structure in which a transparent electrode is divided into a plurality of strip-shaped electrodes, and unit cells formed on the electrodes are connected to each other in series. Under this structure, power loss due to a resistance of the transparent electrode may be reduced. Furthermore, in case of fabricating the integrated-type thin film solar cell in a large area, degradation of conversion efficiency may be reduced.
Hereinafter, a general thin film solar cell will be explained in more details with reference to the attached drawings.
FIG. 1 is a sectional view schematically illustrating a thin film solar cell in accordance with the related art.
As shown, a general thin film solar cell consists of a plurality of unit cells connected to each other in series. Each unit cell consists of a substrate, a transparent electrode 12 formed on the substrate 10, a semiconductor layer 13 formed on the transparent electrode 12 and formed of amorphous silicon, and a metallic electrode 14 formed on the semiconductor layer 13.
The transparent electrode 12 is formed of a Transparent Conductive Oxide (TCO) for transmittance of solar light incident from the outside through the substrate 10.
The semiconductor layer 13 has a pin structure including a p-type silicon layer formed on the transparent electrode 12, an intrinsic silicon layer formed on the p-type silicon layer, and an n-type silicon layer formed on the intrinsic silicon layer.
Hereinafter, processes of fabricating the thin film solar cell will be explained.
Firstly, a transparent electrode forming thin film, a semiconductor layer forming thin film and a metallic electrode forming thin film are deposited on a substrate. Then, a transparent electrode 12, a semiconductor layer 13 and a metallic electrode 14 are formed by a laser scribing process, respectively. Reference numeral H3 indicates a slot formed by a laser scribing process for patterning of the metallic electrode 14. And, reference numeral H4 indicates a slot formed to expose the substrate 10 along four surfaces of an outer periphery portion of the thin film solar cell for insulation of the thin film solar cell from the outside.
Among various methods for forming unit cells in fabricating the thin film solar cell, the aforementioned laser scribing process is being spotlighted in the aspect of efficiency and production costs.
FIGS. 2A to 2D are sectional views sequentially illustrating processes of fabricating the thin film solar cell of FIG. 1.
As shown in FIG. 2A, a transparent electrode forming thin film is formed on a transparent substrate 10. Then, first slots (H1) are formed in the transparent electrode forming thin film by using a laser irradiation device. As a result, transparent electrodes 12 are formed in respective cells so as to be spacing from each other (‘P1’ process).
As shown in FIG. 2B, a semiconductor layer forming thin film is formed on the transparent substrate 10 having the transparent electrodes 12 formed thereon. Then, second slots (H2) are formed in the semiconductor layer forming thin film by using a laser irradiation device. As a result, semiconductor layers 13 are formed in respective cells so as to be spacing from each other (‘P2’ process).
As shown in FIG. 2C, a metallic electrode forming thin film is formed on the transparent substrate 10 having the semiconductor layers 13 formed thereon. Then, metallic electrodes 14 of a left cell (refer to FIG. 2C) are connected to the transparent electrodes 12 of a right cell (refer to FIG. 2C). Then, third slots (H3) are formed in the semiconductor layers 13 and the metallic electrode forming thin film by using a laser irradiation device. As a result, metallic electrodes 14 are formed in respective cells so as to be spacing from each other (‘P3’ process).
As shown in FIG. 2D, laser irradiation is performed along four surfaces of an outer periphery portion of the thin film solar cell where the metallic electrodes 14 have been formed, thereby removing the transparent electrodes 12, the semiconductor layers 13 and the metallic electrodes 14 on the substrate 10. As a result, formed are fourth slots (H4) for insulating the thin film solar cell from the outside (‘P4’ process).
Among various methods for forming unit cells in fabricating the thin film solar cell, the aforementioned laser scribing process is being spotlighted in the aspect of efficiency and production costs.
However, the laser scribing process may have the following disadvantages. More concretely, each process requires separate equipment, and is the ‘P4’ process requires additional equipment. This may increase installation costs and lower spatial efficiency due to added equipment.